1. Field of the Invention
Polystyrene-block-poly(ethylene-ran-butylene)-block-polystyrene (der-PS) has the possibility to generate an ionic bond between, for example, the sulfonic acid groups and basic nitrogens in medetomidine. This is a feature of particular interest in attempts to develop an efficient antifouling surface and improve the performance of antifouling paints with regard to distribution and fixation of the biocide in a paint matrix for even release and effect in hindering for example barnacle colonization. Equivalent systems can be used with the same polymer interaction for even release of a biocide in other paints than marine antifouling paints.
2. Description of the Related Art
The growth of biofouling organisms on underwater structures is a costly and hazardous problem in both marine and freshwater applications. The presence of fouling organisms such as barnacles, algae, tube worms and the like causes economic damage in various ways: for example attachment to the hulls of ships reduces fuel efficiency and causes loss of profitable time because of the need to clean the hulls. Similarly, the attachment of these organisms to cooling water equipment decreases heat conductivity, which eventually reduces the cooling power of the equipment and drives up costs. Also other marine industries and installations, e.g. aqua culture equipment and oil/gas off-shore installations and plants have significant problems with marine biofouling.
Mechanical cleaning of marine surfaces has been introduced as an alternative to toxides and biocides. Notably, water jet cleaning and mechanical cleaning using brushes are in use. The majority of these methods are, however, work-intensive and therefore expensive.
The most efficient antifouling paints have been “self-polishing copolymer” paints based on a polymeric binder to which biocidal organotin, in particular tributylin are chemically bound and from which biocidal organotin is gradually hydrolyzed by seawater as described for example in UK patent GB-A-1457590. These organotin copolymer paints prevent fouling by releasing the organotin compounds during hydrolysis of the polymer. The outermost paint layer becomes depleted of biocides and is swept of the surface of the hull by the movement of the ship through water. Organotin copolymer paints also contain copper oxide pigment which is effective against biofouling against marine organisms while the tributylin acts as a protection from slime and weed.
Paint containing organotin compounds, in particular tributylin have proven to cause negative environmental consequences, harming sea life, causing deformations in oysters and sex changes in whelks. It have been noted that organotin compounds are degraded slowly, and as a consequence these compounds have been accumulated in the sediments in localized areas. Several countries and international organizations have therefore introduced restrictions and prohibitions over their use and further restrictions are expected. Sale and application of tributylin antifouling is to cease, under the International Maritime Organization (IMO) Antifouling System Convention agreed in October 2001. The treaty calls for the ban on application from 1 Jan. 2003 and total prohibition on hulls by 1 Jan. 2008.
With the recent restrictions on the use of these toxic coatings in many countries, the boat and ship owners have fallen back to the technically inferior but less toxic copper oxide based coatings. The life of copper oxide based coatings rarely exceeds 2 years in normal fouling conditions compared to 5 years with self-polishing tributylin. Dissatisfaction existed because copper oxide based coatings did not satisfy the boat and ship operators and owners. Neither did it satisfy environmental protection organizations because of its toxicity to the environment. When the copper compounds are used in reduced concentrations for ecological reasons these paints need however booster biocides against barnacles and algae to achieve performance acceptable for ship owners and other types of marine industries.
Recent progress within the field of self-polishing paints includes the use of a zinc acrylate copolymer utilizing ion exchange as the release mechanism.
Concern for the possible effects of antifoulant toxicants on the environment has encouraged the development and use of systems which attempt to control fouling through surface modifications; for example, prevention of attachment through the use of silicone or fluorine containing polymers having non-stick or release properties, described for example in the following patent documents WO-0014166A1, US92105410, JP53113014, US92847401, DE2752773, EP874032A2, and EP 885938A2. It has been shown that these paints tend to be fragile, resulting in cracking and peeling of the surface.
A new alternative technology was introduced early in the 1990s. Although this was also said to be self-polishing technology, the process to obtain this was no longer through hydrolysis of a polymer. Instead combinations of different water sensitive and partly water soluble binders such as rosin, alone or mixed with acrylates as described in e.g. European patent EP0289481, EP526441 were used. The experience has shown that these paints have not been able to provide the same high and reliable performance as the hydrolyzing organotin-based paints.
Lately new polymers have been developed, based on the same principles as the organotin polymers, i.e. hydrolysis of an insoluble polymer to provide a slightly water soluble product. Among these are e.g. the self-polishing polymers described in WO8402915. Instead of incorporating organotin groups in the polymer chain, this describes the incorporation of organosilyl groups. Experience has shown that these paints have many of the properties associated with the organotin copolymer technology. However, it has also been found that over a long period cracking and peeling on the surface these paints may occur. This is caused by the leaching of soluble components, resulting in the formation of a residual layer that has a different composition than the original paint.
An approach to solve this has been to modify the silyl polymer with different co-monomers, described in EP0646630, EP1016681 and EP1127902. Another approach has been to include fibres to strengthen and increase the cohesive strength in the whole paint and particularly all the residual layer formed as described in WO0077102. A third approach has been to develop a paint wherein mixtures of organosilyl copolymers and rosin have been used to reduce the build up of this residual layer. This has been described in EP0802243. The use of low molecular plasticizers, more specifically chloroparaffines, has also been employed. This is described in EP0775733. Also sulfonic acids have previously been used in antifouling paints, such as in U.S. Pat. No. 6,627,675, but then to improve the self polishing characteristics of the paint, but not coupled to a biocide for even release as in the present invention.
Along the Swedish west coast as well as along the coasts of the North Atlantic Ocean, barnacles and algae are an economic and technical problem. The fully grown barnacle is a stationary crustacean, characterized by a centimeter sized cone shape and enclosing layers of calcinous plates. The mechanical strength of the animal's attachment to solid surfaces is very high, which is why it is difficult to mechanically remove barnacles from solid surface. The animal undergoes different development stages as free-swimming larvae, where the last larva stage is referred to as the cyprid stage. The cyprid screens solid surfaces suitable for settling with the help of a nervous protuberance. In connection with settling, the “settling-glue” referred to as balanus cement is secreted from specialized glands localised to the protuberance and the animal thereby settles to the solid surface. After settlement the animal undergoes a metamorphosis into an adult and stationary animal. When using an old copper leaking paint, with high concentration of copper, one of the first organisms to foul is barnacles.
Algae are also relatively insensitive to copper and the amount of leaking copper needed to inhibit fouling of algae is high. Therefore, copper-containing marine antifouling paints are “boosted” by some manufacturers with more specific algicides. The algicides inhibit the zoospores to attach or inhibit the photosynthesis. Both methods give the result of reduced algae fouling.
A future antifouling paint, boosted with a biocide, should act with high specificity i.e. only target fouling organisms being affected, leaving other marine mechanisms unharmed. The paint should also be designed to attain a controlled release of the active substance. An efficient approach to accomplish a controlled release is by the formation of a bond to a large molecule. Due to a large size and low mobility of a large molecule the biocide diffusion through the paint film can be restricted and thereby have a release rate which is only dependent on the polishing rate of the self-polishing paint. Furthermore the biodegradation of the antifouling agent is another important aspect in order to prevent accumulation in water and sediments and thus affecting the marine environment rather than the target biofouling organism alone.
Several compounds have been presented with antifouling activity. Among those compounds are pharmacological agents with known pharmacological profiles in vertebrates. It has been reported that a selection of pharmacological compounds, that act upon serotonin and dopamine neurotransmitters has the ability to either impede or promote the attachment of barnacles. Serotonine antagonists, such as Cyproheptadine and Ketanserin, and dopamine agonists, such as R (−)-NPA and (+)-Bromocriptine, have exhibited inhibitory properties. Another pharmacological agent that has proven to be an efficient inhibitor with regards to barnacle settlement is the highly selective alpha2-adrenoreceptor agonist Medetomidine or (S,R)-4(5)-[1-(2,3-dimethylphenyl)ethyl]-1H-imidazole. The larval settlement is impeded already at low concentrations, 1 nM to 10 Nm. Medetomidine belongs to a new class of alpha2-receptor agonists containing a 4-substituted imidazole ring with, high selectivity towards 2-adrenoreceptors. Receptors affected by catecholamine neurotransmitters, such as norepinephrine and epinephrine, are termed adrenergic receptors (or adrenoceptors) and can be divided into alpha- and beta-subclasses. The alpha2-adrenoreceptors are involved in the autoinhibitory mechanism of neurotransmitter release and play a significant part in the regulation of hypertension (high blood pressure), bradycardia (reduced heartbeat rate) and even regulation of alertness and analgesia (reduced sensitivity to pain). Medetomidine has been studied in human clinical trials and has also been used as anaesthetics for animals with the (S)-enantiomer, Dexmedetomidine, being the active component.
It is therefore an object of the invention to provide a method of binding biocides in paints using Medetomidine bound to a modified polymer backbone to better control release of the biocide. Other objects and advantages will be more fully apparent from the following disclosure and appended claims.